1. Field of the Invention
The present invention relates generally to a display device using an optical modulator and, more particularly, to a display device using an optical modulator equipped with a lens system having an improved numerical aperture, which considerably reduces the numerical aperture of the lens system in the condensation of +1st order and −1st order diffracted light formed by the optical modulator.
2. Description of the Related Art
With the development of microtechnology, Micro-Electro-Mechanical Systems (MEMS) devices and small-sized equipment, into which MEMS devices are assembled, are attracting attention.
A MEMS device is formed on a substrate, such as a silicon substrate or a glass substrate, in microstructure form, and is a device into which an actuator for outputting mechanical actuating force and a semiconductor Integrated Circuit (IC) for controlling the actuator are electrically or mechanically combined. The fundamental feature of such a MEMS device is that an actuator having a mechanical structure is assembled in part of the device. The actuator is electrically operated using Coulomb's force between electrodes.
FIGS. 1 and 2 illustrate the constructions of representative optical MEMS devices that utilize the reflection or diffraction of light and are applied to optical switches or light modulation devices.
The optical MEMS device 1 illustrated in FIG. 1 includes a substrate 2, a substrate electrode 3 formed on the substrate 2, a cantilevered beam 6 configured to have an actuation electrode 4 that is disposed opposite and parallel to the substrate electrode 3, and a support 7 configured to support one end of the cantilevered beam 6. The cantilevered beam 6 and the substrate electrode 3 are electrically insulated from each other by a gap 8.
In the optical MEMS device 1, the cantilevered beam 6 is displaced by electrostatic attractive force or electrostatic repulsive force generated between the cantilevered beam 6 and the substrate electrode 3 depending on electrical potential applied between the substrate electrode 3 and the actuation electrode 4. For example, as illustrated by the solid and dotted lines of FIG. 1, the beam 14 is alternately displaced in directions inclined with respect to, and parallel to, the substrate electrode 3.
An optical MEMS device 11 illustrated in FIG. 2 includes a substrate 12, a substrate electrode 13 formed on the substrate 12, and a beam 14 formed across the substrate electrode 13 in bridge form. The beam 14 and the substrate electrode 13 are electrically insulated from each other by a gap 10.
The beam 14 includes a bridge member 15 configured to have a bridge shape and made of, for example, an SiN film, and an actuation electrode 16 supported by the bridge member 15 to be opposite and parallel to the substrate electrode 13, made of an Al film having a thickness of 100 nm and configured to function as a reflecting film also. The substrate 12, the substrate electrode 13 and the beam 14 may employ the same constructions and materials as those of FIG. 1. The beam 14 is constructed in a so-called bridge form, in which both ends thereof are supported.
In the optical MEMS device 11, the beam 14 is displaced by electrostatic attractive force or electrostatic repulsive force generated between the beam 14 and the substrate electrode 13 depending on electric potential applied between the substrate electrode 13 and the actuation electrode 16. For example, as illustrated by the solid and dotted lines of FIG. 2, the beam 14 is alternately displaced in directions depressed towards, and parallel to, the substrate electrode 3.
The optical MEMS devices 1 and 11 may be used as optical switches that are provided with switch functions in such a way as to radiate light onto the surfaces of actuation electrodes 4 and 16 which also function as reflecting films and detect reflected light having one direction based on the fact that the reflected directions of light are different depending on the actuated positions of the beams 6 and 14.
Furthermore, the optical MEMS devices 1 and 11 may be used as optical modulation devices for modulating the intensity of light. When the reflection of light is utilized, the intensity of light is modulated using the amount of reflected light per unit time in one direction by vibrating the beam 6 or 14. This optical modulation device employs so-called spatial modulation.
FIG. 3 illustrates the construction of a Grating Light Valve (GLV) device that was developed by Silicon Light Machine (SLM) Co. as a light intensity conversion device for a laser display, that is, a light modulator.
The GLV device 21, as illustrated in FIG. 3, is constructed in such a way that a shared substrate electrode 23, made of a high-melting-point metal, such as tungsten or titanium, and the nitride film thereof, or a polysilicon film, is formed on an insulated substrate 22, such as a glass substrate, and a plurality of beams 24, in the present embodiment, six beams 24 (241, 242, 243, 244, 245 and 246), are arranged parallel to each other across the substrate electrode 23 in a bridge form. The constructions of the substrate electrode 23 and each of the beams 24 are the same as those of FIG. 2. That is, as illustrated in FIG. 3B, the beam 24 is formed by forming an actuation electrode, configured to have an Al film about 100 nm thick, and also configured to function as a reflecting film, on the surface of the bridge member parallel to the substrate electrode 23.
The beams 24, which include bridge members, and actuation side electrodes configured to be disposed on the bridge members and also to function as reflecting films, are commonly called “ribbons”.
When a small amount of voltage is applied between the substrate electrode 23 and the actuation side electrodes also functioning as reflecting films, the beams 24 move toward the substrate electrode 23 due to the above-described electrostatic phenomenon. In contrast, when the application of the voltage is stopped, the beams 24 are separated from the substrate electrode 23 and return to the initial positions thereof.
In the GLV device 21, the heights of the actuation side electrodes are alternately changed by an operation in which the plurality of beams 24 approach or are separated from the substrate electrode 23 (that is, the approach or separation of the plurality of beams 24) and the intensity of light reflected by the actuation side electrodes is modulated by the diffraction of light (a single light spot is radiated onto a total of six beams 24).
The dynamic characteristics of the beams activated using electrostatic attractive force and electrostatic repulsive force are determined almost entirely by the physical properties of the SiN film formed using a CVD method or the like, and the Al film chiefly functions as a mirror.
FIG. 4 illustrates an example of an optical device using a GLV device that is an optical modulation device to which the above-described MEMS device is applied. In this example, the case of application to a laser display is described.
A laser display 51 related to the present embodiment is used as, for example, a large screen projector, particularly a digital image projector, or as an image projection device for a computer.
The laser display 51, as illustrated in FIG. 4, includes a laser light source 52, a mirror 54 positioned with respect to the laser light source 52, an illumination optical system (lens group) 56, and a GLV device 58 functioning as an optical modulation device.
Furthermore, the laser display 51 further includes a mirror 60 for reflecting laser light the intensity of which has been modulated by the GLV device 58, a Fourier lens 62, a filter 64, a diffuser 66, a mirror 68, a Galvano scanner 70, a projection optical system lens group 72, and a screen 74.
In the laser display 51 based on the conventional technology, laser light emitted from the laser light source 52 passes through the mirror 54, and enters the GLV device 58 from the illumination optical system 56.
Furthermore, laser light is spatially modulated by being diffracted by the GLV device 58, is reflected by the mirror 60, and is separated according diffraction order by the Fourier lens 62. Then, only signal components are extracted by the filter 64.
Thereafter, the image signal is decreased in laser spectrum by the diffuser 66, is propagated into space by the Galvano scanner 68 synchronized with the image signal, and is projected into the screen 72 by the projection optical system 70.
According to the conventional technology, when the interval between the diffraction gratings of the diffractive optical modulator decreases, a diffraction angle increases. Accordingly, the Numerical Aperture (NA) of the lens system located in the rear end of a projection lens or the like increases.